What's in a Pulse?

VOL. 8 NO. 2 FEBRUARY 2006 140 At the recent annual American Heart Association meeting, interest in information gathered from taking the pulse resurfaced with the presentation of the preliminary results of the Conduit Artery Function Evaluation (CAFÉ) trial.1 In this AngloScandinavian Cardiovascular Outcomes Trial (ASCOT) substudy,2 similar standard brachial blood pressure reduction was achieved with different classes of antihypertensive agents in hypertensive patients, yet there were differences in cardiac events. The CAFÉ substudy measured the central blood pressure (i.e., the pressure experienced within the aorta), which may help explain these results. In this and the next Common Questions column we will cover two aspects of the pulse in which information can be obtained from peripheral pulses, noninvasively. The pulse wave velocity (PWV) is covered in this column, and the profile or shape of the pulse waveform in the next. The PWV measurement determines the speed at which the pulse wave travels along a length of artery. It is based on the measurement of two pulse waves at two different positions of known distance apart. The carotid and the femoral arteries are frequently used as determinants. These capture the more proximal and the more distal ends of the aorta. Using a surface electrocardiogram as a timing marker, the pulse transit time between these points (e.g., the times in milliseconds between the tip of the QRS and the onset of the pulse waveform obtained at the arterial sites) and the distance measured between the two locations, PWV can be calculated as distance/time. This is typically expressed in meters/second. For example, assume you have measured the distance between the carotid and femoral artery sites and it is 650 mm. Assume next that the difference in time from the QRS complex to the onset of the wave is 80 msec. Dividing the distance (650 mm) by the time difference (80 msec) yields a value of 8.1 m/s. A fairly normal range of aortic PWV is 6–9 m/s. The PWV is thought to be a reflection of arterial stiffness, with faster velocities representing stiffer, less compliant vessels. In many studies, aortic PWV has proven to be a useful index to assess arteriosclerosis and is an independent predictor of cardiovascular events.3 Two observations help explain the association between increased PWV and cardiovascular outcomes. According to the Framingham Heart Study,4 systolic blood pressure rises and diastolic blood pressure falls as large artery stiffness increases in middle-aged and elderly subjects. The raised systolic pressure contributes to left ventricular hypertrophy while the decrease in diastolic blood pressure potentially reduces the coronary perfusion pressure, which contributes to ischemia. Although the stiffness of large arteries increases with age, this is less pronounced in people who engage in regular endurance exercise.5 Regular aerobic exercise reduces arterial stiffness, in contrast to resistance training (e.g., weight lifting), which increases aortic stiffness. Medications appear to differ in their effects on arterial stiffness; however, no published study to date (of which we are aware) has specifically targeted the PWV as a treatment parameter. In the next column, we will cover central aortic pressure and tie in how the central aortic pressure profile is related to arterial stiffness. From the Department of Medicine, Hypertension Program, University of Pennsylvania School of Medicine, Philadelphia, PA Address for correspondence: Raymond R. Townsend, MD, Department of Medicine, Hypertension Program, University of Pennsylvania School of Medicine, White Building, 3400 Spruce Street, Philadelphia, PA 19104 C o m m o n Q u e s t i o n s a n d A n s w e r s i n t h e M a n a g e m e n t o f H y p e r t e n s i o n